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Wednesday, December 18, 2019

Challenges for November 2019 SecTalks London

Selfie by SethBahl from flickr (CC-NC)

October 2019 London SecTalks CTF was jointly won by me and Laurence, so we wrote challenges for November together.

There was a total of 9 challenges, 5 written by me and 4 by Laurence. It was a bit much, as one of the challenges got zero solves until after the event. In spite of very generous hints given halfway through the event.

Challenge files and code used to generate them are available on github.

There are no answers below, but some serious hints which might make it too easy.

For previous rounds, see posts about September 2017November 2017May 2018July 2018October 2018February 2019April 2019, and July 2019 CTFs.

Rockstar (5 points)

The puzzle was a rock song / program in Rockstar programming language. Just running the program would print ASCII codes, which could be easily turned into a flag.

Think Outside The Box (10 points)

The challenge is a BMP image file with flag hidden by incorrect image size. Changing the header reveals the image.

Dots (15 points)

This was Unicode Braille, but dots and no-dots were simply 1s and 0s to be decoded, ignoring any "Braille" structure. To my surprise nobody solved it even when given basically full explanations of what they're supposed to do.

CRC32 MAC (20 points)

A signed cookie server, which uses CRC32 with secret key as MAC. CRC32 is extremely unsuitable in this role.

Decryption Engine (25 points)

A small Python program which uses default password based on current date, and encoded file. Assuming password wasn't passed, the space of possible keys is tiny.

Salted MAC (30 points)

A signed cookie server, which uses SipHash with secret key, and then salt just to be extra sure.

In principle this should be difficult to break. The problem is a trivial implementation mistake.

PHPain (40 points)

It was a separately hosted PHP application vulnerable to recently published CVE.

Poly-DIY MAC (50 points)

Another signed cookie server, using a simplified variant of Poly1305. The variant is reasonable, however the server isn't very careful about nonce handling, and such schemes tend to fail catastrophically when nonce is reused.

GeoCaptcha (60 points)

It's a web "Captcha" which asks the user to locate 20 major cities on the map, with any mistake failing the Captcha.

You can play it here (for a while).

It's worth so many points mostly due to its very high entertainment value, not due to difficulty.

I was curious how people would approach it - only 2 players solved it with code (as robots), 6 solved it with tedious manual process (as humans). So I guess it works as a Captcha.